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    <p>Since this answer is long, here's the summary: <em>No, <code>file_get_contents()</code> is not atomic as it does not respect advisory locks</em>.</p> <h2>About file locks in PHP:</h2> <p>In PHP, while on a *nix platform, filesystem locking is advisory only. Per <a href="http://php.net/manual/en/function.flock.php" rel="nofollow noreferrer">the docs</a> (Emphasis mine):</p> <blockquote> <p>PHP supports a portable way of locking complete files in an advisory way (which means all <strong>accessing programs have to use the same way of locking or it will not work</strong>). By default, this function will block until the requested lock is acquired; this may be controlled (on non-Windows platforms) with the LOCK_NB option documented below.</p> </blockquote> <p>So, as long as all of the processes that are accessing the file use this method of locking, you're fine.</p> <p>However, if you're writing a static HTML file with a sane webserver, the lock will be ignored. In the middle of the write, if a request comes in, Apache will serve the partially written file. The locks will have no effect on the other process reading the lock.</p> <p>The only real exception is if you use the special mount option of <code>-o mand</code> on the filesystem to enable mandatory locking (but that's not really used much, and can have a performance penalty).</p> <p>Have a read on <a href="http://en.wikipedia.org/wiki/File_locking" rel="nofollow noreferrer">File Locking</a> for some more information. Namely the section under <em>Unix</em>:</p> <blockquote> <p>This means that cooperating processes may use locks to coordinate access to a file among themselves, but programs are also free to ignore locks and access the file in any way they choose to.</p> </blockquote> <p>So, in conclusion, using <code>LOCK_EX</code> will create an advisory lock on the file. Any attempt to read the file will block only if the reader respects and/or checks for the lock. If they do not, the lock will be ignored (since it can be). </p> <p>Try it out. In one process:</p> <pre><code>file_put_contents('test.txt', 'Foo bar'); $f = fopen('test.txt', 'a+'); if (flock($f, LOCK_EX)) { sleep(10); fseek($f, 0); var_dump(fgets($f, 4048)); flock($f, LOCK_UN); } fclose($f); </code></pre> <p>And while it's sleeping, call this:</p> <pre><code>$f = fopen('test.txt', 'a+'); fwrite($f, 'foobar'); fclose($f); </code></pre> <p>The output will be <code>foobar</code>...</p> <h2>About <code>file_get_contents</code> specifically:</h2> <p>To your other specific question, first off, there is no <code>LOCK_EX</code> parameter to <a href="http://us.php.net/manual/en/function.file-get-contents.php" rel="nofollow noreferrer"><code>file_get_contents</code></a>. So you can't pass that in.</p> <p>Now, looking at the <a href="https://github.com/php/php-src/blob/master/ext/standard/file.c#L521" rel="nofollow noreferrer">source code</a>, we can see the function <code>file_get_contents</code> defined on line 521. There are no calls to the internal function <code>php_stream_lock</code> as there are when you pass <code>file_put_contents('file', 'txt', LOCK_EX);</code> defined on line 589 of the same file.</p> <p>So, let's test it, shall we:</p> <p>In file1.php:</p> <pre><code>file_put_contents('test.txt', 'Foo bar'); $f = fopen('test.txt', 'a+'); if (flock($f, LOCK_EX)) { sleep(10); fseek($f, 0); var_dump(fgets($f, 4048)); flock($f, LOCK_UN); } fclose($f); </code></pre> <p>In file2.php:</p> <pre><code>var_dump(file_get_contents('test.txt')); </code></pre> <p>When run, <code>file2.php</code> returns immediately. So no, it doesn't appear that <code>file_get_contents</code> respects file locks at all...</p>
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    1. POshould LOCK_EX on both read & write be atomic?
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    1. USedigu
    1. USircmaxell
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    1. POshould LOCK_EX on both read & write be atomic?
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    1. COwill caching implemented in this way (file_get/put_contents with LOCK_EX) be atomic? (cache :: get(), cache :: set()) - while not touching these files from other places?
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      1. USKamil Tomšík
    2. COi realized that before your edit (and added comment that flock has to be used)
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      1. USKamil Tomšík
    3. CO@ircmaxell, why do you do flock($f, LOCK_UN) outside of the `if` statement? Why unlock when it isn't locked in the first place?
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